FIG. 1 shows a vacuum system having a chamber 12, with volume V0, and a pressure control system 10 for controlling the pressure in the chamber. A vacuum pump 14 having a pumping speed Spump is connected to a chamber outlet 16 via a duct 18 for evacuating gas from chamber 12. A valve 20 with variable conductance Cvalve controls gas flow from chamber 12 to pump 14. Valve 20 is usually a throttle valve, as shown, having a moveable vane for changing Cvalve. A pressure gauge 22 monitors the pressure P in chamber 12 and a pressure control unit 24 controls Cvalve according to the monitored pressure P. The flow of process gas at a mass flow rate Qin into chamber 12 is controlled by mass flow controller 26. Process gas is evacuated from chamber 12 at a mass flow rate Qout, which is determined by the product of the pressure in the chamber and the effective pumping speed Seff. The effective pumping speed is:
      S    eff    =      1                  1                  S          pump                    +              1                  C          System                    +              1                  C          Valve                    where Csystem is the conductance of the vacuum system upstream of the valve.
In operation, pump 14 evacuates chamber 12 to a predetermined low pressure and pressure control system 10 incrementally increases the pressure in chamber 12 to allow each processing step to be performed at its required pressure. Pressure change occurs according to:
            ∂      P              ∂      t        =                    Q        In            -              Q        out                    V      0      
Accordingly, when Cvalve is decreased to a predetermined conductance, the effective pumping speed is decreased, and therefore Qout decreases. A reduction in Qout means that the mass of gas in the chamber increases, and therefore chamber pressure increases. It should be noted though that since the chamber pressure increases when Seff decreases (and Qout=pressure×Seff), the rate of change of chamber pressure decreases until it stabilises at a steady pressure. Accordingly, a reduction in Cvalve to a predetermined conductance causes the pressure to increase and stabilise at a set pressure. The change in Cvalve required for each incremental increase in pressure can be predetermined by experimentation or by modelling. Cvalve is changed by altering the position of, for instance, a vane of the valve mechanism.
The time taken for a pressure increase to occur depends on the time it takes for valve 20 to change to a predetermined conductance; and the rate of pressure increase for a fixed valve position and process gas mass flow rate (i.e. the time taken for the pressure to increase once the valve has been changed to the predetermined conductance). The first factor is dependent on the specific design of the valve and is typically very low (less than two seconds). The second factor is dependent on the mass flow rate into the system (Qin), the mass flow rate of the system (Qout) and the volume of the chamber V0.
For instance, as shown in FIG. 2, if a chamber pressure of 2.5 mbar is required for a specific processing step, the valve is set to a preset position so that Cvalve is decreased from 850 m3/hour to 50 m3/hour. Consequently, pressure P increases from 0.1 mbar to 2.5 mbar. In the example shown in FIG. 2, V0 is relatively small and Qin is relatively high, and therefore pressure response time is limited by the response time of the valve. Accordingly, the pressure in the chamber reaches the required pressure of 2.5 mbar after 2 seconds i.e. the time taken for the valve 20 to reach its preset position.
However, if V0 is relatively high or Qin is relatively low, pressure response time is increased. One type of chamber where V0 is generally large is a chamber used for manufacturing flat panel displays. An example of slow pressure response time in a chamber is shown in FIG. 3. FIG. 3 shows pressure increase for a system where V0 is 100 litres and Qin is 2 standard litres per minute. It will be seen that valve 20 reaches its preset position after about two seconds, but pressure P does not reach the required pressure until about 35 seconds.
It is desirable to provide a pressure control system and method capable of reducing pressure response times, particularly in chambers where V0 is relatively high or Qin is relatively low.